The present invention relates generally to structured surfaces fabricated using microreplication techniques. The invention has particular application to structured surfaces that comprise retroreflective cube corner elements.
The reader is directed to the glossary at the end of the specification for guidance on the meaning of certain terms used herein.
It is known to use microreplicated structured surfaces in a variety of end use applications such as retroreflective sheeting, mechanical fasteners, and abrasive products. Although the description that follows focuses on the field of retroreflection, it will be apparent that the disclosed methods and articles can equally well be applied to other fields that make use of microreplicated structured surfaces.
Cube corner retroreflective sheeting typically comprises a thin transparent layer having a substantially planar front surface and a rear structured surface comprising a plurality of geometric structures, some or all of which include three reflective faces configured as a cube corner element.
Cube corner retroreflective sheeting is commonly produced by first manufacturing a master mold that has a structured surface, such structured surface corresponding either to the desired cube corner element geometry in the finished sheeting or to a negative (inverted) copy thereof, depending upon whether the finished sheeting is to have cube corner pyramids or cube corner cavities (or both). The mold is then replicated using any suitable technique such as conventional nickel electroplating to produce tooling for forming cube corner retroreflective sheeting by processes such as embossing, extruding, or cast-and-curing. U.S. Pat. No. 5,156,863 (Pricone et al.) provides an illustrative overview of a process for forming tooling used in the manufacture of cube corner retroreflective sheeting. Known methods for manufacturing the master mold include pin-bundling techniques, laminate techniques, and direct machining techniques. Each of these techniques has its own benefits and limitations.
In pin bundling techniques, a plurality of pins, each having a geometric shape such as a cube corner element on one end, are assembled together to form a master mold. U.S. Pat. No. 1,591,572 (Stimson) and U.S. Pat. No. 3,926,402 (Heenan) provide illustrative examples. Pin bundling offers the ability to manufacture a wide variety of cube corner geometries in a single mold, because each pin is individually machined. However, such techniques are impractical for making small cube corner elements (e.g. those having a cube height less than about 1 millimeter) because of the large number of pins and the diminishing size thereof required to be precisely machined and then arranged in a bundle to form the mold.
In laminate techniques, a plurality of plate-like structures known as laminae, each lamina having geometric shapes formed on one end, are assembled to form a master mold. Laminate techniques are generally less labor intensive than pin bundling techniques, because the number of parts to be separately machined is considerably smaller, for a given size mold and cube corner element. However, design flexibility suffers relative to that achievable by pin bundling. Illustrative examples of laminate techniques can be found in U.S. Pat. No. 4,095,773 (Lindner); International Publication No. WO 97/04939 (Mimura et al.); and U.S. application Ser. No. 08/886,074, xe2x80x9cCube Corner Sheeting Mold and Method of Making the Samexe2x80x9d, filed Jul. 2, 1997.
In direct machining techniques, series of groove side surfaces are formed in the plane of a planar substrate to form a master mold. In one well known embodiment, three sets of parallel grooves intersect each other at 60 degree included angles to form an array of cube corner elements, each having an equilateral base triangle (see U.S. Pat. No. 3,712,706 (Stamm)). In another embodiment, two sets of grooves intersect each other at an angle greater than 60 degrees and a third set of grooves intersects each of the other two sets at an angle less than 60 degrees to form an array of canted cube corner element matched pairs (see U.S. Pat. No. 4,588,258 (Hoopman)). Direct machining techniques offer the ability to accurately machine very small cube corner elements in a manner more difficult to achieve using pin bundling or laminate techniques because of the latter techniques"" reliance on constituent parts that can move or shift relative to each other, and that may separate from each other, whether during construction of the mold or at other times. Further, direct machining techniques produce large area structured surfaces that generally have higher uniformity and fidelity than those made by pin bundling or laminate techniques, since, in direct machining, a large number of individual faces are typically formed in a continuous motion of the cutting tool, and such individual faces maintain their alignment throughout the mold fabrication procedure.
However, a significant drawback to direct machining techniques has been reduced design flexibility in the types of cube corner geometries that can be produced. By way of example, the maximum theoretical total light return of the cube corner elements depicted in the Stamm patent referenced above is approximately 67%. Since the issuance of that patent, structures and techniques have been disclosed which greatly expand the variety of cube corner designs available to the designer using direct machining. See, for example, U.S. Pat. No. 4,775,219 (Appledorn et al.); U.S. Pat. No. 4,895,428 (Nelson et al.); U.S. Pat. No. 5,600,484 (Benson et al.); U.S. Pat. No. 5,696,627 (Benson et al.); and U.S. Pat. No. 5,734,501 (Smith). Some of the cube corner designs disclosed in these later references can exhibit effective aperture values well above 67% at certain observation and entrance geometries.
Nevertheless, an entire class of cube corner elements, referred to herein as xe2x80x9cpreferred geometryxe2x80x9d or xe2x80x9cPGxe2x80x9d cube corner elements, have up until now remained out of reach of known direct machining techniques. A substrate incorporating one type of PG cube corner element is shown in the top plan view of FIG. 1. The cube corner elements shown there each have three square faces, and a hexagonal outline in plan view. One of the PG cube corner elements is highlighted in bold outline for ease of identification. The highlighted cube corner element can be seen to be a PG cube corner element because it has a non-dihedral edge (any one of the six edges that have been highlighted in bold) that is inclined relative to the plane of the structured surface, and such edge is parallel to adjacent nondihedral edges of neighboring cube corner elements (each such edge highlighted in bold is not only parallel to but is contiguous with nondihedral edges of its six neighboring cube corner elements). Disclosed herein are methods for making geometric structures, such as PG cube corner elements, that make use of direct machining techniques. Also disclosed are articles manufactured according to such methods, such articles characterized by having at least one specially configured compound face.
Structured surface articles such as molds or sheetings are disclosed in which a geometric structure has a plurality of faces. At least one of the faces is a compound face comprising a machined portion and a non-machined portion. The non-machined portion can be formed by, for example, replication from another substrate or embossing with a suitable tool. A transition line may separate the machined portion from the non-machined portion. The geometric structure can of course comprise faces arranged to form a cube corner element.
Cube corner elements, and structured surfaces incorporating an array of such elements, are disclosed wherein at least one face of the cube corner element terminates at a nondihedral edge of such element, the face comprising two constituent faces disposed on opposed sides of a transition line that is nonparallel to the nondihedral edge. The cube corner element can comprise a PG cube corner element, and exactly one, exactly two, or all three faces of such element can comprise two constituent faces disposed on opposed sides of a transition line that is nonparallel to the respective nondihedral edge. In an array of neighboring cube corner elements, each cube corner element in the array can have at least one face configured as described above. Further, the cube corner elements can be made very small (well under 1 mm cube height) due to the direct machining techniques employed.
Molds are disclosed in which the structured surface comprises pyramids disposed proximate to at least one cavity, the cavity being formed by non-machined faces and the pyramids being formed at least in part by machined faces.
Methods are disclosed for making a structured surface article comprising at least one geometric structure. The method includes providing a prepared substrate having a non-machined face, and removing material from the prepared substrate to form a machined face such that the machined face and the non-machined face together form one of the faces of the geometric structure.
Methods are also disclosed for forming in a substrate a structured surface that extends along a reference plane and contains PG cube corner elements. The method includes providing a prepared substrate, and forming groove side surfaces in the prepared substrate that extend along axes that are substantially parallel to the reference plane. Faces formed by the groove side surfaces together with other faces incorporated in the prepared substrate combine to form the PG cube corner elements.
Various methods are disclosed for providing the prepared substrate. One such method begins by forming an array of non-machinable protrusions (or cavities) in a first initial substrate, whether by mechanical, chemical, electromagnetic, or other suitable means. A negative copy of the first initial substrate is made in a second initial substrate composed of a material suitable for machining. Upper portions of the protrusions in the second initial substrate are machined to form pyramids. A negative copy of the second initial substrate is then made to form the prepared substrate. The prepared substrate includes cavities corresponding to the pyramids formed in the second initial substrate, and also includes protrusions between the cavities. In some embodiments, the pyramids are cube corner pyramids and the cavities are thus cube corner cavities.
With such a prepared substrate, groove side surfaces are then formed by selectively machining the protrusions in such a way that the machined faces formed in the protrusions are in substantial alignment with faces (xe2x80x9cnon-machined facesxe2x80x9d) of neighboring cube corner cavities, which faces had been replicated from the second initial substrate. At least one face of the PG cube corner elements is a compound face that includes both one of the machined faces and one of the non-machined faces. The compound face may include a transition line that separates the replicated face from the machined face. Retroreflective sheeting or other cube corner articles can then be replicated from the prepared substrate as modified by the direct machining operations.
Another method for providing the prepared substrate begins with the prepared substrate having a substantially flat working surface. An array of cube corner cavities is then formed deep in the working surface by embossing with a hardened tool. Intermediate portions of the working surface forming protrusions between the cube corner cavities are left unfinished. Groove side surfaces are then formed selectively in the protrusions to form machined faces, the machined faces and the embossed faces of the cube corner cavities together forming the array of cube corner elements.